This invention relates to effluent gas treatment, especially treatment of effluent gases obtained from the production of aromatic carboxylic acids such as terepthalic acid.
The invention has application for example to the catalytic combustion of a high pressure effluent gas stream containing combustible components.
In a known process, disclosed in Japanese Kokai 55-99517, for use in the production of terephthalic acid an effluent gas containing combustible and corrosive components such as acetic acid, methyl acetate, p-xylene and carbon monoxide is subjected to catalytic combustion while still at high pressure and the treated gas is then passed through a gas turbine to recover energy which can then be used elsewhere in the terephthalic acid production process. The known method is said to render all combustible and corrosive components of the effluent gas innocuous and the treated gas is exhausted to atmosphere.
Significantly, JP-A-55-9517 makes no reference to any bromine constituent in the effluent gas. However, a commonly used process for the production of terephthalic acid involves the liquid phase oxidation of p-xylene in a carboxylic acid solvent such as acetic acid in the presence of a heavy metal catalyst system including a bromine constituent as a promoter. Where bromine is present, the high pressure effluent gas obtained from the oxidation reaction will contain bromine, mainly in the form of methyl bromide. Methyl bromide is toxic and, if discharged into the atmosphere, is believed to deplete atmospheric ozone. It is therefore important to avoid discharge of methyl bromide into the atmosphere.
If a scheme such as that disclosed in JP-A-55-99517 is used to treat effluent gas containing methyl bromide, the catalytic oxidation will be effective to convert at least some of the methyl bromide into bromine and hydrogen bromide (HBr), components which are potential corrosion-producing agents especially if expensive corrosion resistant materials are to be avoided for fabrication of equipment downstream of the catalytic oxidation zone. The absence of any reference to methyl bromide in JP-A-55-99517 suggests that the terephthalic acid production route either did not employ bromine as a catalyst component or else involved some form of removal system prior to catalytic oxidation or the use of expensive corrosion resistant materials for the construction of equipment handling the effluent gas.
It is believed that in a process already in use bromine and HBr resulting from the catalytic oxidation of effluent gas stream containing, inter alia, methyl bromide are eliminated prior to passage of the treated gas through a gas turbine by scrubbing the pressurised treated gas stream to remove these components. Whilst this is an effective way of ensuring that the materials used in the fabrication of the gas turbine are not exposed to the corrosive bromine components from the gas, the step of scrubbing the gas stream is inevitably accompanied by reduction in the temperature and pressure of the treated gas stream. Consequently, the energy recoverable from the gas stream is reduced.